Method for manufacturing immersion nozzle less susceptible to deposition of alumina for use in continuous casting

ABSTRACT

Disclosed is a method of producing an immersion nozzle for continuous casting, which comprises integrally molding a first zirconia-graphite compound applied to a powder-line portion of the nozzle, and a second compound applied to at least an inner hole portion of the nozzle. The second compound includes 10 mass % or more of clinker particles each containing CaO as a mineral phase, and the surface of at least a part of the clinker particles is subjected to an anti-hydration treatment, such as a heat treatment to be performed under a CO 2  atmosphere to convert CaO to CaCO 3 . The method of the present invention can suppress volume expansion caused by the reaction between CaO and water released from resin added as binder, so as to prevent the occurrence of cracks during a burning process of the molded piece.

TECHNICAL FIELD

The present invention relates to a method of producing an immersionnozzle for continuous casting, and more particularly to a method ofproducing an anti-alumina-buildup immersion nozzle with an inner holewhose wall surface serving as a molten-steel flow passage is formed of aCaO-containing material to prevent buildup of alumina thereon.

BACKGROUND ART

With increasingly strict quality control demanded of steel products, thebuildup of non-metallic inclusions, such as alumina, on the wall surfaceof an inner hole of a submerged or immersion nozzle has a seriousdisadvantage for steel production. Therefore, a good deal of effort hasbeen made to reduce the buildup of nonmetallic inclusions, such asalumina, on the wall surface of an inner hole of an immersion nozzle forcontinuous casting (hereinafter referred to as “immersion nozzle” or“nozzle”).

As one of measures against the alumina buildup, there has been known atechnique in which argon gas is injected from the inner surface of anozzle into molten steel to prevent the alumina buildup in physicalmanner. In this technique, if the argon gas is injected excessively,bubbles of the injected argon gas will be incorporated into molten steelto form pinholes in slabs. Thus, due to the restriction in the allowableinjection amount of argon gas, this method cannot be fully employed as asufficient measure to prevent alumina buildup.

There has also been known a technique intended to provide ananti-alumina-buildup function to refractories themselves. For example,Japanese Patent Laid-Open Publication No. 57-71860 discloses a castingnozzle using refractories including a combination of graphite and eitherone of sintered calcia, fused calcia and another ceramic materialcontaining a CaO component. This invention is intended to induce areaction between CaO contained in the refractories and alumina attachedon the wall surface of a nozzle inner hole so as to form alow-melting-point or fusible substance to prevent the alumina buildup.

Such CaO-containing refractories exhibit a certain level ofanti-alumina-buildup effect. In particular, refractories using a clinkercontaining CaO as a mineral phase, such as calcia-magnesia clinkersincluding calcia clinker and dolomite clinker, have an excellent effect.

For this reason, immersion nozzles have been increasingly produced byintegrally molding a compound using a clinker containing CaO as amineral phase, which is applied to only an inner hole portion of animmersion nozzle or an nozzle body including the inner hole portion, anda zirconia-graphite compound applied to a powder-line portion of thenozzle, and then burning the molded piece in a reduction atmosphere.

This production method involves a problem concerning cracks to begenerated at the boundary between the different compositions during theburning process. This would be because phenol resin, which is added tothe CaO-containing refractories at about several % to less than 20% as abinder or for forming a carbon bond in the refractory matrix, decomposesduring heating to release water therefrom, and the resulting hydrationreaction between the water and the CaO contained as a mineral phase inthe clinker causes volume expansion different from the expandingbehaviors in other portions during heating.

Japanese Patent Laid-Open Publication No. 2000-514394 proposes onemeasure against this problem, in which a CaO-containing powder is addedinto a zirconia-graphite compound for use in a powder-line portion so asto equalize expanding behavior in each portion to prevent the occurrenceof cracks. However, the CaO-containing powder added to thezirconia-graphite compound causes deterioration in corrosion resistance,resulting in undesirably deteriorated durability of a powder-lineportion to be formed of the compound.

DISCLOSURE OF INVENTION

In view of the problem in the conventional immersion-nozzle productionmethod comprising integrally molding a zirconia-graphite compoundapplied to a powder-line portion of an immersion nozzle, and a compoundusing a clinker containing CaO as a mineral phase, which is applied toat least an inner hole portion of the nozzle to prevent alumina buildup,and then burning the molded piece in a reduction atmosphere, it istherefore an object of the present invention to provide an improvedimmersion-nozzle production method capable of preventing the occurrenceof cracks to be generate during the burning process.

Through research working towards solving the above conventional problem,the inventor found that one factor of the occurrence of cracks as theproblem in the conventional production method, or the volume expansionarising during the burning process, can be prevented by subjecting theclinker containing CaO as a mineral phase to an anti-hydration treatmentso as to suppress the hydration reaction between the CaO and the waterreleased from the resin. The present invention has been accomplishedbased on this knowledge.

Specifically, the present invention provides a method of producing animmersion nozzle for continuous casting, which has zirconia-graphiterefractories applied to a powder-line portion thereof. The methodcomprises integrally molding the zirconia-graphite refractories, and acompound applied to at least an inner hole portion of the immersionnozzle. The compound includes 10 weight % or more of clinker particleseach containing CaO as a mineral phase, wherein the surface of at leasta part of the particles is subjected to an anti-hydration treatment.

The anti-hydration treatment for the clinker containing CaO as a mineralphase may specifically performed by converting the CaO exposed outsidefrom the surface of the clinker particle, to a compound nonreactive withwater, or by coating the entire clinker particle with a film impermeableto water.

In the former treatment, the CaO may be converted to a water-stablecompound, such as CaCO₃, CaSO₄ or Ca₃ (PO₄)₂. For example, the CaO maybe converted to CaCO₃ by subjecting untreated clinker to a heattreatment under a CO₂ atmosphere at a temperature of 300 to 850° C.

In the latter treatment for coating the entire clinker particle with afilm impermeable to water, the water-impermeable film may be formedusing heat-resistant resin, such as silicone resin; pitch; or magnesiumsulfate.

Among the above anti-hydration treatment, the treatment of convertingthe CaO exposed outside from the surface of the clinker particle, toCaCO₃, is most preferable in view of treatment cost, anti-hydrationperformance, and influence on the quality of the refractories after theburning process.

While it would be preferable to subject all of the clinker particles tothe anti-hydration treatment, only a part of the clinker particles maybe subjected to the treatment as long as an intended effect can beachieved.

The refractory compound comprising calcia-magnesia clinker including adolomite clinker and a calcia clinker containing CaO as a mineral phasemay be a mixture of such clinkers and binder. The refractory compoundmay selectively include any other suitable aggregate, such as CaZrO₃clinker containing CaO without CaO as a mineral phase, materialprimarily consisting of a CaO—SiO₂ clinker and another CaO-basedcompound, oxide, carbide or carbon, etc.

In view of the anti-alumina-buildup function, the amount of the clinkerparticles each containing CaO as a mineral phase is 10 mass % or more,preferably 20 mass % or more, even more preferably 30 mass % or more, onthe basis of 100 mass % of the entire compound. The body of theimmersion nozzle may be made of any suitable conventional material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of a dolomite clinker subjected toan anti-hydration treatment in one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention will now be described inconnection of the Examples.

Table 1 shows the composition of compound including clinker particleseach containing CaO as a mineral phase, the application of a heattreatment performed under a CO₂ atmosphere to convert the CaO exposedoutside from the surface of the clinker particle, to CaCO₃, and theinspection result of the presence of cracks after the burning process.

Each specimen for the testing was prepared by applying azirconia-graphite on the powder-line portion and a compound shown inTable 1 onto the main body or the inner hole portion other than thepowder-line portion, and carrying the whole body to CIP molding under apressure of 1000 Kg/cm², or 98 Mpa, and burning under a reductionatmosphere at a temperature of 1000° C. TABLE 1 Comparative ComparativeExample Inventive Example Example Inventive Example 1 2 3 1 2 3 4 5 6 45 6 Compounding graphite 25 35 35 25 35 35 10 — — 10 — — ratio (mass %)(0.5 mm ≧) dolomite clinker 20 15 15 20 15 15 — — — — — — (1 mm ≧)dolomite clinker 35 30 30 35 30 30 35 40 40 35 40 40 (0.2 mm ≧) dolomiteclinker 20 20 — 20 20 — 35 35 — 35 35 — (0.074 mm ≧) magnesia clinker —— 20 — — 20 — — 35 — — 35 (0.074 mm ≧) calcia clinker — — — — — — 20 2525 20 25 25 (1 mm ≧) Application of CO₂ treatment to NON NON NON YES YESYES NON NON NON YES YES YES clinker *1 Presence of cracks afier burningYES YES YES NON NON NON YES YES YES NON NON NON process

In Table 1, all of the dolomite clinker particles in Inventive Examples1 to 3 were subjected to a heat treatment under a CO₂ atmosphere, or ananti-hydration treatment. The dolomite clinker particles in ComparativeExamples 1 to 3 were conventional clinkers without application of theanti-hydration treatment. The compound to be applied to the portion ofthe nozzle other than the powder-line portion comprises either one ofclinkers as shown in Table 1, which contains CaO as a mineral phase.This compound was prepared by homogenously kneading the clinkerparticles and an appropriate amount of phenol resin added thereto. Thezirconia-graphite compound comprises 90 mass % of zirconia and 10 mass %of graphite. The zirconia-graphite compound was prepared by homogenouslykneading the zirconia, the graphite and an appropriate amount of phenolresin added thereto, and commonly applied to all of the immersionnozzles in Table 1.

FIG. 1 is a schematic sectional view of the dolomite clinker particle asshown in Table 1, which is subjected to an anti-hydration treatment forconverting CaO contained in the clinker particle and exposed outsidefrom the surface of the clinker particle, to CaCO₃.

Each of Inventive Examples 1 to 3 using the clinker particles subjectedto the anti-hydration treatment could avoid the occurrence of cracksduring the burning process. In contrast, all of Comparative Examples 1to 3 had transverse cracks at the boundary between the CaO-containingrefractories and the zirconia-graphite refractories.

Table 1 also shows Inventive Examples 4 to 6 each of which includescalcia clinker particles and dolomite clinker particles which aresubjected to the heat treatment under a CO₂ atmosphere, or theanti-hydration treatment. The dolomite clinker particles in ComparativeExamples 4 to 6 were conventional clinkers without application of theanti-hydration treatment. In Inventive Examples 4 to 6 and ComparativeExamples 4 to 6, the nozzle body was formed of a magnesia-graphitecompound prepared by adding an appropriate amount of phenol resin to 70mass % of magnesia and 30 mass % of graphite, and homogenously kneadingthem. A compound including clinker particles each containing CaO as amineral phase was applied only to the inner hole portion.

As seen in the results of Inventive Examples 4 to 6 and ComparativeExamples 4 to 6, each of Inventive Examples using the clinker particlessubjected to the anti-hydration treatment could avoid the occurrence ofcracks during the burning process. In contrast, all of ComparativeExamples had transverse cracks at the boundary between thezirconia-graphite refractories and the magnesia-graphite refractories, apart of which extended toward the outer periphery of the nozzle.

As mentioned above, the method of the present invention can provide animmersion nozzle for continuous casting, with an excellentanti-alumina-buildup function, while preventing the occurrence of cracksduring the burning process to provide enhanced process yield. Theobtained immersion nozzle can contribute to the improvement inoperational efficiency of continuous casting and in steel quality.

INDUSTRIAL APPLICABILITY

The present invention is applicable to the production of an immersionnozzle for continuous casting, which uses a material containing CaO inthe wall surface of an inner hole of the nozzle serving as amolten-steel flow passage to prevent alumina buildup.

1. A method of producing an immersion nozzle for continuous casting,which has zirconia-graphite refractories applied to a powder-lineportion hereof, said method comprising: integrally molding saidzirconia-graphite refractories, and a compound applied to at least aninner hole portion of said immersion nozzle, said compound including 10mass % or more of clinker particles each containing CaO as a mineralphase, wherein the surface of at least a part of said clinker particlesis subjected to an anti-hydration treatment.
 2. The method as defined inclaim 1, wherein said anti-hydration treatment comprises converting theCaO exposed outside from the surface of said clinker particle, to CaCO₃.